Technical limitations.
IL-2 doesn't know modern Windows' "windowed fullscreen" mode.
Hence, whenever you run IL-2 in fullscreen mode (which usually is the case), it will always try to get hold of the full main screen render area and put itself topmost of any other window whenever a render redraw gets initiated.
Only way out: Run the game in windowed mode.
Second thing is the way IL-2 deals with DirectInput devices (mouse, in this case).
IL-2 knows to mouse modes in the conf.ini [rts] section, "mouseUse=1" and "mouseUse=2", the latter is what most of us play with.
In "mouseUse=1" mode, your mouse can move inside and outside IL-2's window (provided you run IL-2 in windowed mode), hence you can use your mouse there anywhere. On the flipside, the game doesn't recognize any mouse moves once the cursor leaves the render area (in windowed mode) or touches the screen edge (in fullscreen mode), hence you cannot e.g. move your view in circles by moving the mouse in one direction continuously, like you could in the other mode.
In "mouseUse=2" mode, mouse moves are not bound to any screen/render areas, hence you can move your mouse endlessly in any direction and the game will keep recognizing (and applying) that move. On the flipside, once the game gets hold of your mouse, it will only release it on another render update, and only when you deactivate the game "window" (be it windowed or fullscreen) using alt+tab. Ctrl+Alt+Delete doesn't make the game release the mouse hook, hence you cannot move your mouse cursor outside of IL-2 unless you get it to release the mouse using alt+tab.
Mike